This example simulates the command "CHKDSK C: /F:O", but only shows the second half of the information. It returns the number of available allocation units to the initiating process. Some return code checking has been omitted for brevity.

#define INCL_DOSFILEMGR   /* File Manager values */
#define INCL_DOSERRORS    /* DOS Error values    */
#include <os2.h>
#include <stdio.h>

int main (VOID) {
   ULONG   aulFSInfoBuf[40] = {0};         /* File system info buffer     */
   APIRET  rc               = NO_ERROR;    /* Return code                 */

   rc = DosQueryFSInfo(3L,                     /* Drive number 3 (C:)     */
                       FSIL_ALLOC,             /* Level 1 allocation info */
                       (PVOID)aulFSInfoBuf,    /* Buffer                  */
                       sizeof(aulFSInfoBuf));  /* Size of buffer          */

   if (rc != NO_ERROR) {
      printf("DosQueryFSInfo error: return code = %u\n", rc);
      return 1;
   } else {
      printf ("%12ld bytes in each allocation unit.\n",
               aulFSInfoBuf[1] * (USHORT)aulFSInfoBuf[4]);
             /* (Sectors per allocation unit) * (Bytes per sector) */
      printf ("%12ld total allocation units.\n", aulFSInfoBuf[2]);
      printf ("%12ld available allocation units on disk.\n", aulFSInfoBuf[3]);
   }
   DosExit(EXIT_THREAD,aulFSInfoBuf[3]);  /* Return available allocation units
                                             to the initiating process      */
   return NO_ERROR;
}